Control system



Sept. 14, 1943. w. L. McGRATH CONTROL SYSTEM 2 Sheets-Sheet 1 Filed Jan.18, 1941 INVENTOR William I. MQGIORI'LA jaw Ki AITORNEY p 1943- w. L. MGRATH 2,329,636

' CONTROL SYSTEM Filed Jan. 18, 1941 2 Sheets-Sheet 2 /ZZO INVENTORWilliam 1 M Gvmm.

ATTORNEY Patented Sept 14, 1943 CONTROL SYSTEM William L. McGrath,'Philadelphia, Pa., assignor to Minneapolis-Honeywell Regulator Company,Minneapolis, Minn., a corporation of Delaware Application January18,1941, Serial No. 375,009

18 Claims. (Cl. 236-6 8) This invention relates to systems forcontrolling the magnitude of a condition generally, and moreparticularly to thermostatically operated systems for controlling thetemperature of a building.

Most common types of temperature control systems include a thermostat inthe space whose temperature is to be controlled, and a heater controlledby the thermostat. In such systems, the thermostat is usually set toturn 'on the heater whenthe temperature of the space being heated dropsbelow a, certain level, and to turn the heater off when the temperatureexceeds a different, slightly higher level. There is nearly always adelay between a, call for heat by the thermostat and the supplying ofheat to the space, with the result that the space temperature continuesto drop below the desired value. Similarly, when the spacetemperature'has been restored and the thermostat calls for a shutdown of the heatingsystem, usually some heat remains in the heating means which has notbeen dissipated into the space, and the dissipation of this remainingheat usually results in raising the space temperature above the desiredvalue. Conditions such asthese are known as over-shooting or hunting,and their elimination has been greatly sought after in designing controlsystems.

An object of this invention is to provide a control system havingimproved means for preventing the occurrence of over-shooting.

Another object of the present invention is to control heating meansadapted to operate at different stages of heat output in such a manneras to substantially prevent over-shooting.

A further object of the invention is to provide a control system inwhich a two-stage heater is operated either intermittentlyat its lowstage, or alternately at its low and high stages.

A further object of the invention is to provide a control system inwhich a two-stage heater is operated either intermittently at its lowstage, alternately at its high and low stages, or continuously at itshigh stage.

. A further object of the invention is, to provide a control system fora heater operable at a plurality of stages, whereby the average heateroutput may be selectively set at one of a number of levels greater thanthe number ofheater stages. For

' example, a heater may be operable at either low or high output stages.Continuous operation at these two stages produces two different averageoutput levels, while alternate operation at the two levels ill produce athird average output level intermediate of the other two.

A further object of the invention is to provide a control system inwhich a condition changing means may be controlled by closure of asingle set of contacts operated by condition responsive means but inwhich improved means are provided for preventing intermittent operationof the condition changing means due to chattering of the contacts.

A further object is to provide a temperature control system of the typedescribed in which the condition responsive means is an on-and-ofl'thermostat, i. e., one which closes its contacts when operating in onedirection only.

Other objects and advantages of the invention will become apparent froman inspection of the following specification together with theaccompanying drawing, in which Figure 1 is'a diagrammaticshowing of aheating system operated in accordance'with my invention,

Figure 2' is a diagrammatic showing of a modification of the system ofFigure 1, and

. nace I0 is supplied with gas through a valve I2.

An operating mechanism i3 i provided for the valve I 2 and is of a typewhich operates the valve to three different positions, for example,closed, partly open and full open. A pilot burner I4 is supplied withgas through a pipe l5 which by-passes the valve l2. A safety switch itis mounted adjacent the pilot burner I 4 in order that the system may beshut down upon failure of the pilot flame. A thermostatically operatedlimit switch 20 is mounted in the bonnet of the furnace l0 and operatesto shut down the system in case the air temperature within the bonnetrises to an unsafe value.

The furnace I0 is controlled by a thermostat generally indicated at 2|and consisting of a bimetallic element '22 operating a pair of switcharms 23 and 24. As indicated by the legend in the drawings thethermostat 2| moves the switch arms 23 and 24 to the left upon a drop intemperature, thereby sequentially engaging them with stationary contacts25 and 26.

A relay 30 is provided comprising a winding 3i operating switch arms 32and 33. When the winding 3| is energized the switch arms 32 and 33 areengaged with stationary contacts 34 and 35, respectively. A thermalswitch 38 is provided for intermittently opening the energizing circuitof switch arms 47 and ii the relay 3!) for a purpose to be describedlater. The switch 36 comprises a heater coil 31 and a bimetallic switcharm 38 cooperating with a stationary contact 35. The thermal switch 35may conveniently be placed near the relay 30, which is usually locatedin the basement, or some other place remote .from the thermostat, whereplenty of space is available.

Another relay 45 is provided in the control system which comprises awinding 46 operating These switch arms are engaged respectively withcontacts 49 and 50 when the winding 45 is energized. When the winding isdeenergized switch arm 43 engages another stationary contact 5 I.

Power is supplied to the system through. a transformer 52 having aprimary winding 53 and a secondary winding 54.

Operation of Figure 1 species When the parts are in the position shownin the drawings the thermostat 2| is satisfied and the furnace Iii isnot in operation. The furnace is in normal operation condition. That is,the pilot burner is lit so that the safety switch I6 is closed, and thebonnet temperature is normal so that the limit switch 25 is closed. Upona drop in temperature adjacent the thermostat 2| the switch arm 23 willbe closed against the contact 25 thereby completing an energizingcircuit for the relay winding 3 I. This circuit may be traced from theupper terminal of secondary winding 54 through conductors 55 and 55,bimetallic element 22, switch arm 23, contact 25, a-conductor 51,contact 39, switch 36, relay winding 3|, and a conductor 58 to the lowerend of secondary winding 54. Energization ofthe relay winding 3| willcause the closure of switch arm 32 against contact 34, therebycompleting a holding circuit for the relay which is independent of thethermostat 2|. This circuit may be traced from the upper terminal ofsecondary winding 54 through conductor 55, contact 34, switch arm 32,switch 36, winding 3| and conductor 58 back to the lower terminal ofsecondary 54.

Energization of relay 3| will also cause the closure of switch arm 33against contact 35, thereby completing an energizing circuit for thevalve operating mechanism l3 which will cause the valve |2 to move toits intermediate position. This circuit may be traced from the upperterminal of secondary winding 54 through the conductor 55, a conductor50, valve operating mechanism l3, a conductor 3|, contact 5|, switch arm43, a conductor 52, limit switch 20, a conductor $3, safety switch I5, aconductor 64, switch arm 33, contact 35, and conductor 58 back to thelower terminal of secondary winding 54.

The valve operating mechanism I3 is constructed so that the valve isnormally closed when the mechanism is not energized. When an energizingcircuit is completed through the conductor 58 and switch contact 5i thevalve is moved to its intermediate position and when a circuit iscompleted through contact 50 and a conductor 65 the valve is moved toits full open position.

After the last described circuit has been established the furnace itwill continue to operate at its lower stage of output, providing thethermostat 2| does not change its position. This operation at the lowstage of output, however, will be rendered intermittent by the action ofthe switch 35. As soon as the winding 31 has heated up due to the flowof current therethrough the bimetallic element 38 will be warped awayfrom the contact 39 thus opening the energizing circuit of the relaywinding 3|. The energizing circuit for the valve operating mechanism iswill consequently be opened at the contact 35 and the operation of thefurnace will stop until the bimetallic switch arm 38 has cooledsufficiently to reengage the contact 39 at which time the abovedescribed heatin cycle will be repeated.

If, While the intermittent operation of the heater at its lower stage ofoutput is going on, the thermostat 2i should experience a further fallin temperature, the switch arm 24 would be moved to engage the contact25. This would close a circuit shunting the switch 36 from theenergizing circuit of the relay coil 3i. This circuit may be traced fromthe upper end of secondary winding 54 through conductors 55 and 56,bimetallic element 22, switch arm 24, contact 26, a conductor 56, relaywinding 3i, and conductor 58 back to the lower terminal of secondarywinding 54.

Closure of switch arm 24 against contact 26 would also complete anenergizing circuit for relay winding 46. This circuit may be traced fromthe upper end of secondary winding 54 through conductors 55 and 55,bimetallic element 22, switch arm 24, contact 25, a conductor 61, athermal switch 58, relay winding 45, and a conductor 15 to the lower endof secondary winding 54.

Energization of the winding would cause closure of the switch arm 4?against contact 49 thereby completing a holding circuit for the winding46 which is independent of the thermostat 2|. This circuit may be tracedfrom the upper end of secondary winding 54 through conductors 55 and 75,contact 49, switch arm 41, thermal switch 68,

. relay winding 46 and conductor i5 to the lower end of secondarywinding 54.

Energization of relay winding 46 would also cause the switch arm 43 toclose against the contact 5|] thereby shifting the energizing circuit ofthe valve operating mechanism Hi to the conductor and causing the valveto change from intermediate position to full open position. This causesthe furnace H] to operate at its maximum output. Assuming that thethermostat 2| does not change its position this'operation will continueuntil the thermal switch 58 opens, deenergizing the rela 45. This actionwill allow the furnace to go back to its lower stage of output.

It will be seen therefore that the furnace i0 is operated intermittentlyat its low output stage when the switch arm 23 engages the contact 25and that when the switch arm 24 engages contact 26 the furnace isoperated alternately at its low and high output stages.

This mode of operation provides a. much smoother flow of heat to thespace being controlled than is obtained by the use of the prior artdevices. The intermittent operation of the heater prevents the buildingup of an excessive reserve of heat in thesy'stem to be dissipated afterthe thermostat becomes satisfied. The use of atwo-stage heater permits agraduated response of the heater to the two-position thermostat, whileretaining the advantages of intermittin: operation for both positions ofthe thermos a Figure 2 In Figure 2 I have shown a different embodimentof my invention in which a difierent type of thermostat is used and inwhich the furnace may be controlled so as to operate continuouslyfications are merely illustrative of alternative forms of constructionwhich maybe used with either embodiment of my invention.

A furnace 80 has a mainburner 84 supplied with fuel through a pipe 8Icontrolled by a valve 82 having an operating mechanism 83. A pilotburner 85 is provided and a safety switch 86 is mounted adjacentthereto. A thermostat 81 is mounted in the bonnet of a furnace andoperates'a limit switch 88 and a fan switch 89. The

furnace 80 is of the warm air type and is probimetallic element 9|,operating switch arms 92,-

93 and 94. As indicated by the legend in the drawings, the thermostat 90moves the switch arms 92, 93 and 94 to the left upon a drop intemperature, thereby sequentially engaging them with stationary contacts96, 91 and 98.

A relay I is used in the present" embodiment of my invention andcomprises a winding IOI operating switch arms I02 and I03. When'thewinding IN is energized the switch arms I02 and I03 are closed againstcontacts I04 and I05, respectively. A thermal switch I08 is provided tointermittently open the energizing circuitv of the relay winding IN.This switch I06 comprises a heatercoil' I01 and a bimetallic switch armI08 cooperating with a contact I09.

Another relay H0 is used in this'control system having a winding IIIoperating switch arms H2, H3 and H4.

gaged with contacts II5, H6 and 1, respectively. When the winding III isdeenergized switch arms H3 and H4 engage contacts H8 and H9,respectively. A thermal. switch I20. is provided to intermittently openthe energizing circuit of the relay winding III. This thermal switch I20comprises a heater coil HI and a bimetallic switch arm I22 cooperatingwith a contact I23.

This system is supplied with energy from lines I30 and I3I. Atransformer I32 is provided to convert the line voltage to the lowvoltagenecessary for the operation of the relays. This transformer I32has a primary winding I33 and a secondary winding I34.

Operation of the-species of Figure 2 When the parts are in the positionshown in the drawings. the thermostat 90 is satisfied andarm 92, contact96, a conductor I31, thermal switch I06, relay winding: MI, andconductor I38 to the left hand end .of secondarywinding I34.

Energization of the relay winding IlII results When the winding I II isener gized the switch arms H2, H3 and H4 are enin the closure of aholding circuit for the relay through the contact I04 and switch armI02. This circuit may be traced from the right hand end of secondarywinding 134 through conductor I35, contact I04, switch arm I02, thermalswitch I06, relay winding IM and conductor I 38 to the. left hand end ofsecondary winding I34.

Energization of relay winding IOI also closes the switch arm I03 oncontact I05 thereby comsupply line I30.

pleting an energizing circuit for the valve operating mechanism 83. Thiscircuit may be traced from supply line I3I through conductor I39, switcharm, I03, contact I05, a conductor I40, limit switch 88, a conductorI4I, safety switch 85, a conductor I42, valve operating mechanism 83, aconductor I43, contact II8, switch arm I I 3, and conductors I44 and I45to This energizes the operating mechanism 83 so that the valve 82 ismoved to an intermediate position and the furnace 80 is operated at itslower output stage. As long as no further movement of the thermostat 90takes place this operation continues intermittently under the control ofthe thermal switch I06.

If the temperature adjacent the thermostat 90. continues to drop, theswitch arm 93 engages the stationary contact 91; this completes anenergizing circuit for the relay winding IOI relay winding III.

which shunts the thermal switch I08. This circuit-may be traced from theright hand and of secondary winding I34 through conductors I35 and I36,bimetallic element 9I, switch arm 93, contact 97, a conductor I50,relay. winding IN, and conductor I38 to the left hand end of secondarywinding I34.

Closure of the switch arm 91 also completes an energizing circuit forthe This circuit may be traced fromthe right'hand end of secondarywinding I34 through conductors I 35 and I36, bimetallic element 9|,switch arm 93, contact 9], a conductor I5I, thermal switch I20, relaywinding II I, a conductor I 52, and conductor I38 to the switch arm II2to close against the contact II5 thereby completing a holding circuitfor the relay III which is independent of the thermostat 90. Thiscircuit may be traced from the right hand end of secondary winding I34through conductors I35 and I53, c'ontact II5, switch arm II2, thermalswitch I20, relay winding III, and conductors I52 and I38 to thesecondary winding I34. v

Energization of relay winding III also causes the switch arm II3 to moveout of engagement with the contact H8 and close against the contact II6. This causes the energizing circuit for the valve operating mechanism83 to be completed through a conductor I54 instead of through conductorI43 and therebycauses the valve operating mechanism to move the valve toits full open position.

If the thermostat 90 maintains this position, the furnace will beoperated alternately at its high and low output levels under control ofthe thermal switch I20. 3 If the temperature adjacent the thermostatdrops still further, the switch arm 94 will be moved into engagementwith the contact 98.- This completes an energizing circuit for the relaywinding II I which shunts the thermal switch I23 and thereby maintainsthe-heater 80 at its high 93 against contact left hand end ofconductorsI35 and. I36, bimetallic element 9|, switch arm 94, contact 98,conductor I60, relay winding Ii I and conductors I52 and I38 to the lefthand end of secondary winding I34.

When the temperature in the furnace bonnet rises a sufiicient amountthat the air therein may be efliciently used for heating the house, thefan switch 89 will be closed by the thermostatic element 81, thuscompleting an energizing circuit for the fan 95. This circuit may betraced from supply line I3I through conductor E39, fan switch 86, aconductor I65, fan 95, a conductor I62, contact H9, switch arm H4, andconductor I45 to supply line I30. The last described circuit causes oeration of the fan at. it lower speed. When the furnace is operating atits high output stage, the switch arm H4 is moved against contact inand. the fan energizing circuit then passes through a conductor I63.nections of the fan 95 are such that the fan is operated at its highspeed when energized through the conductor I63 and at its. low speedwhen energized through conductor 562.

It will be seen that in this embodiment of my invention, the furnace 80is operated at its low output stage when the switch arm 92 engagescontact 96, alternately at its low and high output stages when switcharm 93 engages contact 91, and continuously at its high output stagewhen switch arm 94 engages contact 98. .It will be further apparent thatthe fan 95 will be operated whenever the bonnet temperature is highenough that the air therein will efficiently heat the house. The speedat which the fan is driven" will depend upon the output stage at whichthe furnace is operating.

Figur 3 In Figure 3 is shown another embodiment of my invention in whichthe two-stage furnace may be operated continuously at either of itstwostages and in which the only function of the timer is to preventintermittent operation of the furnace because of chattering of thethermostat contact.

A furnace I50 has a main burner II supplied with fuel through a pipe I52controlled by a valve I53 having an operating mechanism I54. Theoperating mechanism I54 is preferably of an electrically operated typeand has three terminals I55, I56 and I51. The construction is preferablysuch that when the'mechanism I54 is energized through the terminals I55and. I51, the valve is moved to its intermediate position and when themechanism is energized through the terminals I66 and I51 the valve ismoved to its'full open position. When the operating mechanism I54 isdeenergized the valveis closed by a spring or other internal biasingmeans.

A pilot burner I60 is provided to ignite the gas flowing from themainburner and a safety switch IN is provided adjacent to the pilotburn-. er. A thermostat I62 is mounted in the bonnet of the furnace andoperates a limit switch I63.

The furnace I50 is controlled by a'thermostat generally indicated at I64and consisting of a bimetallic element I65 operating a pair of switcharms I66 and I61. The switch arms I66 and I61 cooperate with a pair ofstationary contacts I68 and I69 respectively. As indicated by the legendin the drawings, the thermostat I64 moves the switch arms I66 and I61 tothe left upon a drop in temperature thereby sequentially engaging themwith the stationary contacts I68 and I69.

A relay I is provided for controlling the operation of the furnace andincludes a winding I16 The internal con-.

I83 and a contact I84.

and switch arms I11 and I18 cooperating with contacts I19 and I80,respectively. A thermal switch I8 I cooperate with the relay I15 andcomprises a heater winding I82, 9. bimetallic element The constructionof switch I8! is such that when heater I82, is energized, the heatsupplied to bimetallic element I83 causes it to warp away from contactI84,

A second relay generally indicated at I90 is provided for controllingthe shift of the burner E from low to high stage operation and viceversa. The relay I90 consists of a winding I9I operating switch arms I92and 93. The switch arms I92 and I93 engage stationary contacts I94 andI95 wh n the relay is energized. When the relay is cleenergized, theswitch arm I93 engages a contact i963. A thermal switch I91, similar. toswitch ESI, cooperates with the'relay I90 and consists of aheaterwinding I 96, a bimetallic element E99 and a contact 206.

The system is supplied with energy through a transformer 20I having aprimary winding 202 and a secondary winding 203.

Operation of species of Figure 3 I When the parts are in the positionshown in the drawings, the thermostat I64 is satisfied and the furnaceI50 is not in operation. If the temperature adjacent the thermostat I64now begins to decrease, the switch arm I66 is moved to the left againstthe contact I68. This completes an energizing circuit for the relaywinding I16 which may be traced from the upper terminal of transformersecondary 203 through a conductor 2I0,

' the switch arm I11 to close against the contact I19. This completes aholding circuit for the relay I15 which may be traced from the upperterminal of secondary winding 203 through conductor 2I0, contact I19,switch arm I11, conductor 2I4, heater element I82, bimetallic elementI83, contact I84, winding I16 and conductor 2I3 to the lower terminal ofsecondary winding 203. It should be noted that the heater I82 is shuntedby the engagement of the switch arm I66 with the contact I68 so that thebimetallic element I83 is not warped so as to disengage from contact I84as long as switch arm I66 remains in engage-- ment with contact I66.

Energization of relay I15 also causes the closure of switch arm I18against contact I thereby completingan energizing circuit for the valveoperating mechanism I 54. This circuit may be traced from the upperterminal of secondary winding 203 through conductor 2I0, a conductor220, terminal I51, operating mechanism I54, terminal I55, a conductor22I, contact I96, switch arm I93, a conductor 222, switch I63, aconductor 223, safety switch I6I a conductor 224, switch arm I18,contact I 80, and conductor 2I3 to the lower end of secondary winding203; Assuming that the furnace I50 is in a safe operating condition, andthat the safety switch I6I and the limit switch I63 are both thereforeclosed, this energizing circuit results in the opening of the valve I53to its intermediate position and the operation of the furnace I50 at itslow stage.

If the temperature adjacent to thermostat I64 continues to fall, theswitch arm I61 is moved into engagement with the contact I69. Thiscompletes an energizing circuit for the relay winding I9I This circuitmay be traced from the upper terminal of secondary winding 293 throughconductors 2I9 and 2| I, bimetallic element. I65, 'switch arm I61,contact I69, a conductor 225, bi-

metallic element I99, contact 299, winding I9I and a conductor 226 tothe lower terminal of secondary winding 293.

Energization of the winding I9I results in the I moving of switch armI92 into engagement with contact I94, thereby completing a holdingcircuit for the winding I9I. This. holdingcircuit may be traced from theupper terminal of secondary winding 293 through conductor 2I9, aconductor 221, contact I94, switch arm I92, heater element I98,bimetallic element I99, contact 299, winding I9I and conductor 226 tothe lower terminal of secondary winding 293. As previously noted in thecase of thermal switch I 8|, the heater element I98 is shunted-by theengagement of switch arm I61 with contact I69 and is therefore notenergized to cause warping of the bimetallic element I99 away fromcontact 299 as long as that switch arm remains in engagement with thecontact I69.

Energization of the winding I9I also causes I I switch arm I93 to moveout of engagement with the contact I96 and into engagement with thecontact I95. This causes the valve operating mechanism I54 to beenergized through a new circuit which may be traced from the upperterminal of secondary winding 293 through conductors 2I9 and 229,terminal -I51, operating.

Now assume that the temperature adjacent the thermostat I64 begins toincrease, the switch arm I61 is moved to the right and disengages fromthe contact I69. This opens the energizing circult of the relay windingI9I. ing I! is then energized through its holding circuit previouslydescribed, including the heater element I96. Energization of the heaterI98 re-' sults after a predetermined time in the warping of bimetallicelement I99 away from contact 299 thereby opening the holding circuit.The winding I9 I is then deenergized and the switch arm I92 moves awayfrom contact I94 and the switch arm I93 moves out of engagement withcontact I95 and into engagement with contact I96.

This opens the circuit which causes the operating mechanism I54 to holdthe valve I53in its full open position and again closes the circuitwhich causes valve I53 to be held in its intermediate position. Theburner I59 therefore returns to its low stage operating condition.

If the temperature continues to decrease, the

switch arm I66 moves out of engagement with.

the contact I68, thus opening the energizing circuit for relay windingI16. The relay winding I16 is then energized through the holding circuitincluding the heater I92 previously described. After this circuitincluding the heater I82 has been energized for. a predetermined periodof time, the bimetallic element I 93 will be warped away from engagementwith the contact I94. This opens the holding circuit for the relay windeing I16 and allows switch arms I11 and I18 to move out of engagementwith contacts I19 and I89 respectively.

Movement of switch arm I19'out of engagement with contact I89 opens theenergizing circuit tothe operating mechanism I54 and allows the valveI53 to return to closed position thereby shutting down the furnace I59.

- It will be seen that fluctuation of the temperature adjacent thethermostat 94 at or near the temperature values which cause engagementof either of the switch arms I66 or I61 with its associated contact doesnot cause intermittent energization of the relay I16. When the switcharm I66 first closes against the contact I68, it completes an energizingcircuit for the relay I 16. As soon as the latter picks up its armatureit completes a holding circuit for itself through the heater I82.Therefore any. fluctuations in the temperature adjacent-the thermostat I64 which might cause the switch arm I66 to vibrate or chatter againstits contact I68, would not cause intermittent energization of the relayI16 but would merely produce intermittent energiza tion of the heaterI82. The heater I92 is designed so that it must remain energized for. apredetermined time before the bimetallic element I83 is heatedsumciently to separate from the contact I84. Therefore, thisintermittent enerheater I82 to warp the bimetallic element I83 The relaywindaway from the contact I84.

The operation of the relay I9I at such times as the switch arm I61chatters against the contact I69 is exactly similar to that described inconnection with the relay I15. It may therefore be seen that chatteringof either thermostat contact will not cause intermittent operation ofthe furnace.

While I have shown and described preferred embodiments of my invention,it should be understood that it is to be limited only by the deiinitionsset forth in the appended claims.

I claim as my invention:

1. A condition control system comprising in combination, conditionchanging means operable at two different stages of activity, meansmovable in accordance with the vaiue of a condition indicative of theneed for operation of said condition changing means, electrical circuitsfor controlling. the stage of activity of said condition changing means,means for intermittently opening said circuits, and connections betweensaid intermittently at one stage when said movable means is in a secondrange of positions, and alternately at its two stages when said movablemeans is in a third range of positions.

2. A condition control system comprising in combination, conditionchanging means operable at two different stages of activity, meansmovable in accordance with thevalue of a condition indicative of theneed for operation of said condition changing means, electrical circuitsfor controlling the level of activity of said condition changing means,means for intermittently opening said circuits, and connections betweensaid movable means and said circuits, said movable means, said openingmeans and said connections cooperating to prevent operation or" saidcondition changing means when said movable means is in a first range ofpositions, to cause operation of said condition changing meansintermittently at one stage when said movable means is in a second rangeof positions, alternately at its two stages when said movable means isin a third range of positions, and continuously at its other stage whensaid movable means is in a fourth range of positions.

3. A temperature control system comprising in combination, temperaturechanging means operable at two different output stages, thermostaticmeans movable in response to a condition indicative of the need foroperation of said temperature changing means, a pair of switchesclosable in sequence by said thermostatic means, electrical circuitscontrolled by said switches for determining the output stage of saidtemperature changing means, means for periodically interrupting saidcircuits, said circuits and said interrupting means causing operation ofsaid temperature changing means intermittently at the lower of said twooutput stages when one of said switches is closed, and alternately atsaid two output stages when both of said switches are closed.

4. A temperature control system comprising in combination, temperaturechanging means operable at two diiferent output stages, thermostaticmeans movable in response to a condition indicative of the need foroperation of said temperature changing means, three switches closable insequence by said thermostatic means, electrical circuits controlled bysaid switches for determining the output stage of said temperaturechanging means, means for periodically interrupting certain of saidcircuits, said circuits and said interruptng means causing operation ofsaid temperature changing means intermittently at the lower of said twooutput stages when one of said switches is closed, alternately at saidtwo stages when two switches are closed, and continuously at the higherof said stages when all three switches are closed.

5. In a cpndition control system, in combination, condition changingmeans, a pair of relays for controlling operation of said conditionchanging means, a. device responsive to a condi-- tion indicative of theneed for operation of said condition changing means, first, second andthird switches closable in sequence by said device, first electricalcircuit means for energizing one of said relays intermittently when saidfirst switch is closed, second circuit means for energizing said onerelay continuously when said second switch is closed, third circuitmeans for energizing the other of said relays intermittently when saidsecond switch is closed, and fourth circuit means for energizing saidother relay continuously when said third switch is closed.

6. In a condition control system, in combination, condition changingmeans, a pair of relays for controlling operation of said conditionchanging means, a device responsive to a condition indicative of theneed for operation of said condition changing means, first and secondswitches closable in sequence by said device, first electrical circuitmeans for energizing one of said relays intermittently when said firstswitch is closed, second electrical circuit means for energizing saidone relay continuously when said second switch is closed, and thirdelectrical circuit means for energizing the other of said relaysintermittently when said second switch is closed.

one of said circuits for each relay being controlled by one of saidswitches and including means to deenergize said relay intermittently,the other of said circuits for each relay being controlled by the switchfollowing said one switch in sequence, and serving to energize saidrelay continuously.

8. A temperature control system comprising in combination, temperaturechanging means operable at two different output stages, thermostaticmeans movable in response to a condition indicative of the need foroperation of said temperature changing means, a pair of switchesclosable in sequence by said thermostatic means, a pair of relays, anenergizing circuit for each relay controlled by one of said switches, aholding circuit for each relay, switches for controlling said holdingcircuits, means for opening each said holding circuit switch apredetermined time after opening of the" corresponding thermostaticallyoperated switch, and means controlled by said relays for determining theoutput stage of said temperature changing means.

9. A temperature control system comprising in combination, temperaturechanging means operable at two difierent output stages, thermostaticmeans movable in response to a condition indicative of the need foroperation of said temperature changing means, a pair of switchesclosable in sequence by said thermostatic means, a pair of relays, aholding switch controlled by each of said relays, an energizing circuitfor each re-' lay including one of said thermostatically controlledswitches, a holding circuit for each relay including its related holdingswitch, current-re: sponsive means in each holding circuit shunted byclosure of the associated thermostatically controlled switch, each saidcurrent-responsive means-being effective to open the circuit through ita. predetermined time after opening of the associated thermostaticallycontrolled switch, and means controlled by said relays for determiningthe output stage of said temperature changing means. i

10. In a condition control system, in combination, condition changingmeans, first switch means, a relay for controlling operation of saidcondition changing means and for operating said first switch means,second switch means, a device responslve to a condition indicative ofthe need for operation of said condition changing means for operatingsaid second switch means,

said second switch means comprising a pair of switches closable insequence by said device, third switch means, time-controlled means forintermittently operating said third switch means, a first energizingcircuit for said relay winding including in parallel said first switchmeans and the first to close of said pair of switches and in.

cluding said third switch means in series, and a secondenergizing'circuit for said relay winding including the second to closeof said, pair of switches and a connection shunting all the switches insaid first circuit.

11. In a condition control system, in combination, condition changingmeans, first switch means, a relay for controlling operation of saidcondition changing means and for operating said first switch means,second switch means, a device responsive to a condition indicative ofthe need for operation of said condition changing means for operatingsaid second switch means, third switch means, current responsive meansfor intermittently operating said third switch means, an energizingcircuit for said relay including said second switch means and said thirdswitch means in series, and a holding circuit for said relay includingsaid first switch means and said current responsive means in aconnection shunting said second switch means.

12. A temperature control system comprising in combination, temperaturechanging means, thermostatic means movable in response to a conditionindicative of the need for operation of said temperature changing means,a switch operable by said thermostatic means upon movement thereof, arelay for controlling said temperature changing means, a holding switchoperated by said relay, a normally closed heat responsive switch,electrical heater means for opening said switch, an energizing circuitfor said relay including said thermostatically operated relay energizedfor a predetermined time after opening of said energizing circuit, saidholding circuit including said holding switch, said electrical heatermeans, and said heat responsive switch, said thermostatically operatedswitch serving to shunt said heater means so as to maintain said relaycontinuously energized while said thermostatically operated switch isclosed and to delay the operation of said heater means during a periodwhen the thermostatically operated switch is chattering.

13. A condition control system, comprising in combination, conditionchanging means, a switch operable in response to a condition indicativeof the need for operation of said condition changing means, a relay forcontrolling said condition changing means, a holding switch operated bysaid relay, a normally closed switch, current responsive timer means foropening said normally closed switch, an energizing circuit for saidrelay including said condition responsive switch, and a holding circuitfor maintaining said relay energized for a predetermined time afteropening oi said energizing circuit, said holding circuit including saidholding switch, said current responsive timer means and said normallyclosed switch. said condition responsive switch being connected so as toshunt said current responsive timer means, thereby maintaining saidrelay continuously energized while said condition re-.

sponsive switch is closed and delaying op ration of said timer meansduring a period when said condition responsive switch is chattering.

14.1 condition control system comprising in combination, conditionchanging means operable at a plurality oi diflerent output stages, meansmovable in response to a. condition indicative of the need for operationof said condition chang ing means, a plurality of switches closable insequence by'said condition responsive means, at least one electricalcircuit controlled by each said load circuit means, a second relay forcontrolling the other of said load circuit means, first control circuitmeans for energizing said first relay, first current responsivecircuit-interrupting means connected in said first control circuitmeans, second controlv circuit means for energizing said first relayindependently of said circuitinterrupting means, at least-one controlcircuit means for energizing said second relay, and second currentresponsive circuit-interrupting means connected in said one controlcircuit means for said second relay,

16. Electrical control apparatus,.comprising in combination, a pair ofload circuit means to be controlled, a first relay for controlling oneof said load circuit means, a second relay for controlling the other ofsaid load circuit means, first switch, and a holding circuit formaintaining said control circuit means for energizing said first relay,first intermittently operated circuit-interrupting means connected insaid first control circuit means, second control circuit means includingtwo parallel branches, one branch for energizing said fist relayindependently of said circuit-interrupting means and the other branchfor energizing said second relay, and second intermittently operatedcircuit-interrupting means connected in said otherbranch.

17. A condition control system comprising in combination, conditionchanging means operable at two difierent stages of activity, meansresponsive to a condition indicative of the need (or operation ofsaid'condition changing means and efi'ective'to produce two difierentcontrol eifects sequentially upon a unidirectional change in saidcondition in a direction indicative of increased need for operation ofsaid condition changing means, means responsive to the first of said twocontrol efiects to cause intermittent operation of said conditionchanging means at its lower stage. and means responsive to the second ofsaid two control eilects to cause said condition changing means tooperate alternately at its higher and lower stages.

18. A condition control system comprising in combination, conditionchanging means operable at two'difi'erent stages of activity, meansresponsive to a condition indicative of the need for operation of saidcondition changing means and eil'ective to produce three difierentcontrol effects sequentially upon a unidirectional change in'saidcondition in a direction indicative of increased need for operation ofsaid condition changing WILLIAM L. McGRATH.

